For reducing an operation load imposed on a driver, a vehicle speed control system is known. This system carries out a constant speed driving control operation under which a driver can drive a vehicle with keeping a vehicle speed set by the driver under such a condition that the vehicle speed must be kept to a constant speed, for example, when the vehicle runs on an express highway or the like. The constant speed driving control operation means a control operation for driving a vehicle with keeping a set vehicle speed by controlling an acceleration device for accelerating the vehicle and a deceleration device for decelerating the vehicle.
Various kinds of control systems have been proposed to perform the vehicle driving based on the vehicle speed control with higher safety. For example, there is known a control system for controlling (i.e., reducing) the vehicle speed so that the vehicle can pass over a curve under a stable condition when the vehicle runs at the curve (JP-A-2002-96654).
However, if the vehicle speed control described above is actually carried out, the vehicle might be decelerated to a vehicle speed at which it can stably pass over a curve before the vehicle reaches the curve (i.e., in front of the entrance of the curve).
When a vehicle approaches a curve, a driver controls the vehicle speed as follows. That is, the driver brakes the vehicle to decelerate the vehicle while immediately estimating a vehicle speed, a vehicle running locus, etc. under which the vehicle can safely run at the curve in consideration of achievable information on the shape of the curve, the current speed of the vehicle, the load surface condition, the surrounding conditions of the vehicle and the curve, the performance of the vehicle, etc., and estimating required deceleration by the time when the vehicle goes into the curve.
When the vehicle approaches the curve, the driver continues to drive the vehicle by properly adjusting the deceleration degree through braking operation or the like while estimating a vehicle speed at which the vehicle can stably run at the curve peak corresponding to the tightest position of the curve and estimating required deceleration by the time when the vehicle reaches the curve peak. When the vehicle has passed over the curve peak, the driver waits for arrival of the vehicle at the exit of the curve while keeping the vehicle speed, and then accelerates the vehicle until the vehicle speed reaches a legal speed for the road or a speed at which the driver believes that the vehicle runs safely on the next straight load.
As described above, in some cases, the vehicle speed control carried out by the conventional vehicle speed control system is different from the vehicle speed control carried out by general drivers. Thus, the drivers have a sense of incongruity. Furthermore, when a vehicle runs behind another vehicle, the following vehicle may excessively approach to the preceding vehicle if the preceding vehicle is decelerated to the vehicle speed at which the preceding vehicle can safely pass over a curve before the preceding vehicle reaches the curve.
Furthermore, various other kinds of control systems have been also proposed to perform the vehicle driving based on the vehicle speed control with higher safety. For example, there is known a control system in which the radius of curvature of a curve is calculated on the basis of plural nodes of a map data base and a permissible speed of approach to the curve is set on the basis of the radius of curvature thus calculated to perform the vehicle speed control (U.S. Pat. No. 6,138,084).
However, it has been found that the radius of curvature of a curve calculated in the manner as descried above is frequently deviated from the arcuate shape of the actual curve. That is, there are some curves which are not designed in a uniformly arcuate shape like it is drawn by compasses, but designed in an arc-composite shape like it is achieved by combining some arcuate shapes different in radius of curvature. Therefore, as described above, when the radius of curvature is calculated on the basis of the nodes of the map data base, the arc drawn on the basis of the radius of curvature thus calculated is not necessarily coincident with the shape of the actual curve. This is more remarkable as the shape of the curve is more complicated.
Furthermore, all the nodes of the map data base which serve as standard points for the calculation of the radius of curvature do not necessarily indicate the center (center points) of the road. They are located with some dispersion in the width direction of the road. This causes the difference between the arc drawn on the basis of the calculated radius of curvature and the shape of the actual curve. Therefore, the conventional vehicle speed control system for setting the permissible approach speed to the curve on the basis of the calculated radius of curvature carries out the vehicle speed control on the basis of the radius of curvature which is deviated from the shape of the actual curve.
A general driver immediately estimates a vehicle speed, a vehicle running locus, etc. under which the vehicle can safely run at the curve in consideration of achievable information on the shape of the actual curve, the current speed of the vehicle, the load surface condition, the surrounding conditions of the vehicle and the curve, the performance of the vehicle, etc., thereby controlling the vehicle. Accordingly, in some cases, vehicle speed control of the conventional vehicle speed control system for setting the permissible approach speed to the curve on the basis of the radius of curvature may be different from the vehicle speed control carried out by the general driver. Thus the driver has a sense of incongruity.